Diffuser pipe with splitter vane

ABSTRACT

A compressor diffuser for a gas turbine engine includes one or more diffuser pipes having a tubular body defining an internal flow passage extending therethrough. The tubular body includes a first portion extending in a first direction, a second portion extending in a second direction different from the first direction, and a curved portion fluidly linking the first portion and the second portion. A splitter vane is disposed within the internal flow passage of the curved portion of the tubular body, the splitter vane defining a convergent flow passage between itself and a radially inner wall of the curved portion.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, moreparticularly, to compressor diffusers therefore.

BACKGROUND

Diffuser pipes are provided in certain gas turbine engines for directingflow of compressed air from an impeller of a centrifugal compressor toan annular chamber containing the combustor, while diffusing the highspeed air. The diffuser pipes are typically circumferentially arrangedat a periphery of the impeller, and are designed to transform kineticenergy of the flow into pressure energy. Diffuser pipes may provide auniform exit flow with minimal distortion, as it is preferable for flamestability, low combustor loss, reduced hot spots etc.

While longer diffuser pipes may accomplish better diffusion, spatialconstraints of the gas turbine engine may restrict their length. Largeflow diffusion in diffuser pipes over insufficient pipe length mayresult in thick and weak boundary layer buildup on the wall of thediffuser pipe. To compensate for a shorter length, many diffuser pipeshave a tight bend. Turbulence and other non-streamline behavior of theflow at the bend may lead to pressure losses and decrease efficiency ofthe diffuser pipe, and therefore of the compressor.

SUMMARY

There is therefore provided a compressor diffuser for a gas turbineengine comprising: at least one diffuser pipe having a tubular bodydefining an internal flow passage extending therethrough, the tubularbody including a first portion extending in a first direction, a secondportion extending in a second direction different from the firstdirection, and a curved portion fluidly linking the first portion andthe second portion; and a splitter vane disposed within the internalflow passage of the curved portion of the tubular body, the splittervane defining a convergent flow passage between itself and a radiallyinner wall of the curved portion.

There is also provided a method for diffusing fluid flow in acompressor, comprising: conveying fluid flow through a diverginginternal flow passage of a compressor diffuser, the internal flowpassage including at least one curved portion; splitting the fluid flowinto first and second fluid passages defined within the internal flowpassage, the first and second fluid passages being at least partiallylocated within the curved portion; and converging fluid flow through thefirst fluid passage.

There is further provided a centrifugal compressor, comprising: animpeller having an inner hub with a plurality of vanes extendingtherefrom, the impeller being rotatable within an outer shroud about acentral longitudinal axis, the impeller having a radial impeller outlet;and a diffuser configured to diffuse gas received from the impelleroutlet, the diffuser comprising: at least one diffuser pipe having atubular body defining a flow passage extending therethrough, the tubularbody including a first portion extending in a first direction, a secondportion extending in a second direction different from the firstdirection, and a curved portion fluidly linking the first portion andthe second portion; and a splitter vane disposed within the internalflow passage of the curved portion of the tubular body, the splittervane defining a convergent flow passage between itself and a radiallyinner wall of the curved portion.

There is alternately provided a compressor diffuser for a gas turbineengine comprising: a plurality of diffuser pipes each having a divergingtubular body defining a flow passage extending fully therethrough, thetubular body including a first portion extending in a first direction, asecond portion extending in a second direction different from the firstdirection, and a curved portion interconnecting the first portion andthe second portion; and a plurality of splitter vanes each extendinginto the flow passage of a corresponding diffuser pipe and disposed atleast partially within the curved portion between an inner and an outerwall thereof, the splitter vane extending a length between a leadingedge and a trailing edge, a first side of the splitter vane convergingalong the length toward one of the inner and outer walls, and an opposedsecond side of the splitter vane diverging along the length away fromthe other one of the inner and outer walls.

There is alternately provided a method for diffusing fluid flow,comprising: conveying fluid flow through a diverging tubular body havinga first portion extending in a first direction, a second portionextending in a second direction different from the first direction, anda curved portion interconnecting the first portion and the secondportion; splitting fluid flow at the curved portion of the tubular bodyinto a first fluid passage and a second fluid passage; and convergingfluid flow through the first fluid passage, and diffusing fluid flowthrough the second fluid passage.

There is alternately provided a centrifugal compressor, comprising: animpeller having an inner hub with vanes thereon and rotatable within anouter shroud about a central longitudinal axis, the impeller having aradial impeller outlet; and a diffuser operable to diffuse gasesreceived from the impeller outlet, the diffuser comprising: a pluralityof diffuser pipes each having a diverging tubular body defining a flowpassage extending fully therethrough, the tubular body including a firstportion extending in a first direction, a second portion extending in asecond direction different from the first direction, and a curvedportion interconnecting the first portion and the second portion; and asplitter vane extending into the flow passage and disposed at leastpartially within the curved portion between an inner and an outer wallthereof, the splitter vane extending a length between a leading edge anda trailing edge, a first side of the splitter vane converging along thelength toward one of the inner and outer walls, and an opposed secondside of the splitter vane diverging along the length away from the otherone of the inner and outer walls.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a perspective view of an impeller and a correspondingplurality of radially disposed diffuser pipes of a centrifugalcompressor for a gas turbine as shown in FIG. 1;

FIG. 3 is a perspective view of a diffuser pipe of the compressor ofFIG. 2 having a splitter vane, according to an embodiment of the presentdisclosure;

FIG. 4 is a partially sectioned perspective view of the diffuser pipe ofFIG. 3;

FIG. 5 is a detailed, partially sectioned, perspective view of thediffuser pipe of FIG. 3;

FIG. 6 is a schematic cross-sectional view of a diffuser pipe having asplitter vane and an air recirculation conduit, according to anotherembodiment of the present disclosure; and

FIG. 7 is a side elevation view of the diffuser pipe of FIG. 3, shown atleast partially transparent with shading to illustrate streamlines offluid flow therethrough.

DETAILED DESCRIPTION

FIG. 1 illustrates a gas turbine engine 10 of a type preferably providedfor use in subsonic flight, generally comprising in serial flowcommunication along an engine axis 11: a fan 12 through which ambientair is propelled, a compressor section 14 for pressurizing the air, acombustor 16 in which the compressed air is mixed with fuel and ignitedfor generating an annular stream of hot combustion gases, and a turbinesection 18 for extracting energy from the combustion gases. Thecompressor section 14 may include a plurality of stators 13 and rotors15 (only one stator 13 and rotor 15 being shown in FIG. 1), and it mayinclude a centrifugal compressor 19.

The centrifugal compressor 19 of the compressor section 14 includes animpeller 17 and a plurality of diffuser pipes 20, which are locateddownstream of the impeller 17 and circumferentially disposed about aperiphery of the exit of the impeller 17. The diffuser pipes 20 converthigh kinetic energy at the impeller 17 exit to static pressure byslowing down fluid flow exiting the impeller. The diffuser pipes 20 mayalso redirect the air flow from a radial orientation to an axialorientation (i.e. aligned with the engine axis 11). In most cases, theMach number of the flow entering the diffuser pipe 20 may be at or nearsonic, while the Mach number exiting the diffuser pipe 20 may be in therange of 0.2-0.25 to enable stable air/fuel mixing, and light/re-lightin the combustor 16.

Turning now to FIG. 2, the impeller 17 and the plurality of diffuserpipes 20, also referred to as “fishtail diffuser pipes”, of thecentrifugal compressor 19 are more clearly seen. Each of the diffuserpipes 20 includes a diverging (in a downstream direction) tubular body22, formed, in one embodiment, of sheet metal. The enclosed tubular body22 defines a flow passage extending the length of the diffuser pipe 20through which the compressed fluid flow is conveyed. The tubular body 22includes a first portion 24 extending generally tangentially from theperiphery of the impeller 17. An open end is provided at an upstream endof the tubular body 22 and forms an inlet of the diffuser pipe 20. Thefirst portion 24 is inclined at an angle θ1 relative to a radial axis R.The angle θ1 may be at least partially tangential, or even substantiallytangentially, and may further correspond to a direction of fluid flow atthe exit of the blades of the impeller 17, such as to facilitatetransition of the flow from the impeller 17 to the diffuser pipes 20.The first portion 24 of the tubular body 22 can alternatively extendmore substantially along the radial axis R.

The tubular body 22 of the diffuser pipes 20 also includes a secondportion 26, which is disposed generally axially and is connected to thefirst portion 24 by an out-of-plane curved portion 28 or “bend”. An openend at the downstream end of the second portion 26 forms an outlet ofthe diffuser pipe 20. Preferably, but not necessarily, the first portion24 and the second portion 26 of the diffuser pipes 20 are integrallyformed together and extend substantially uninterrupted between eachother, via the curved, bend, portion 28.

The large swirl of the flow exiting the impeller 17, and thereforeentering the first portion 24 of each of the diffuser pipes 20, may beremoved by shaping the diffuser pipe 20 with the curved portion 28, suchthat the flow is redirected axially before exiting to the combustor 16.For a given impeller 17 exit Mach number and swirl of the flow, theeffectiveness of a diffuser pipe may be dependent upon its length. For afishtail pipe type diffuser, such as the one described herein, thegreater its length the easier it is for the pipe to diffuse flowefficiently without, or with only minimal, flow separation at the curvedportion 28. Effective length can be obtained by extending the piperadially, axially, or both. Longer diffuser pipes are however lessdesirable, in that they may potentially increase both the weight and thesize of the engine. In addition, a required gap between the outlet ofeach diffuser pipe 20 and the location of the combustor fuel nozzles isanother constraint that may place physical restrictions on radial/axialextension of the diffuser pipes 20. As a result, the diffuser pipe 20may be designed to have a tight 90 degree bend 28 to compensate for itsreduced length.

Referring now to FIG. 3, the tubular body 22 of each diffuser pipe 20has a radially inner wall 28 a and a radially outer wall 28 b, whichmeet to form an enclosed fluid passage 29 extending through the lengthof the tubular body 22. The radially inner wall 28 a corresponds to thewall of the tubular body 22 that has the smallest turning radius at thecurved portion 28, whereas the radially outer wall 28 b corresponds tothe wall of the tubular body 22 that has the largest turning radius atthe curved portion 28.

As noted above, and as can be seen in FIG. 3, the tubular body 22diverges in the direction 27 of fluid flow F therethrough, in that theinternal fluid passage 29 defined within the tubular body 22 increasesin cross-sectional area along its length which extends between an inlet23 and an outlet 25 thereof. This increase in cross-sectional area ofthe internal fluid passage 29 through each diffuser pipe 20 may becontinuous along the complete length of the tubular body 22 or thecross-sectional area of the internal passage may increase in gradualincrements along the tube length. In the depicted embodiment, thecross-sectional area of the inner fluid passage 29 defined within thetubular body 22 increases gradually and continuously along its length,from the inlet 23 to the outlet 25. The first portion 24 may have agenerally circular cross-sectional shape, while the second portion 26may have a flattened oval (or oblong) cross-sectional shape. Other typesof cross-sections for the first portion 24 and the second portion 26 arehowever also within the scope of the present disclosure.

Still referring now to FIG. 3, each of the diffuser pipes 20 includes aninternal airfoil shaped flow guide or splitter vane 30 (hereinaftersimply “splitter vane”), disposed within the internal fluid passage 29and extending between opposed inner side walls of the tubular body 22.The splitter vane 30 is disposed within the internal fluid passage 29 ata length-wise point between the inlet 23 and the outlet 25 of thetubular body 22 forming the diffuser pipe 20, although as seen in FIG.3, in the depicted embodiment the splitter vane 30 is located at or nearan intermediate point in the diffuser pipe 20 within the bend 28.

The splitter vane 30 is oriented in the diffuser pipe 20 so that aleading edge 32 of the splitter vane 30 receives the incoming fluid flowF, and in at least the present embodiment the airfoil-shaped splittervane 30 curves in a same direction as the curved bend portion 28 of thediffuser pipe 20. The splitter vane 30 is generally disposed to conformto the fluid flow F (i.e. streamlined) so that there is minimalseparation when the fluid flow F encounters the splitter vane 30. Thesplitter vane 30 divides, or “splits”, incoming fluid flow F within theupstream portion of internal fluid passage 29 into two separate flowpassages over the length of the splitter vane 30.

As noted, the splitter vane 30 in the present embodiment is disposedwithin the internal flow passage 29 within the curved or bent portion 28of the diffuser pipe 20. The curved portion 28 may be defined by a zoneof redirection between the first portion 24 and the second portion 26 ofthe pipe, as illustrated by the two dotted lines joined by the bracket28 in FIG. 3. The splitter vane 30 may however only be partiallydisposed within this curved portion 28, and therefore may also extend,at least partially, into the first and/or the second portion 24, 26 ofthe diffuser pipe 20. In one embodiment, a majority of the total lengthof the splitter vane 30 is disposed within this redirection zone 28defined at the curved portion 28 of the diffuser pipe. In the embodimentof FIG. 3, the entirety of the length of the splitter vane 30 isdisposed within this redirection zone 28 defined at the curved portion28 of the diffuser pipe 20. The presence of the splitter vane 30 may atleast reduce some of the drawbacks associated with the tight bend of thecurved portion 28, as noted below.

It has been found that the curvature of the curved portion 28 of thediffuser pipe 20 may cause the flow to detach from the internal surfacesof the inner and/or outer walls 28 a, 28 b, which can result in pressurelosses and non-uniform flow at the outlet 25 of the diffuser pipe 20.Mixing losses may also occur and negative effect overall diffuserperformance. Such flow separation in the diffuser pipe, beginning at thecurved portion, may not only be potentially detrimental to theperformance and operability of the compressor section, but also to itsstructural integrity as flow separation can be destructive in nature andcan lead to premature pipe breakage, fatigue, cracking, noise, flameinstability etc.

In order to at least partially help address these issues, the presenceof the splitter vane 30 in the diffuser pipe 20 of the presentdisclosure may relieve the pressure gradient at the curved portion 28,and therefore reduce the occurrence of flow separation downstream of thecurved portion of the pipe. This may accordingly help reduce aerodynamicpipe loses and may therefore contribute to improved overall compressorperformance (e.g. stall enhancement, improved surge margin) and range.While it is possible that the splitter vane 30 may cause some smalladditional amount of aerodynamic friction loss, the reduction in overallmixing losses enabled by the splitter vane 30 may more than offset thisincrease in friction loss.

Still referring to FIG. 3, the splitter vane 30 of the present diffuserpipe 30 is airfoil shaped and includes a leading edge 32 and a trailingedge 34. The extent of the splitter vane 30 between its leading andtrailing edges 32,34 defines its length. The splitter vane 30 has afirst side 36 and an opposite second side 38, each of which faces one ofthe inner and outer walls 28 a,28 b of the tubular body 22. Thedescriptors “first” and “second” do not limit the sides 36,38 to theparticular configuration shown in the figures. It will be appreciatedthat the first side 36 may face either one of the inner and outer walls28 a,28 b, and that the second side can similarly face either one of theinner and outer walls 28 a,28 b. For the sole purpose of facilitatingunderstanding, the first side 36 will be described herein as a“convergent” side where fluid flow is converged over the length of thesplitter vane 30, and the second side 38 will be described as a“divergent” or “diffusing” side where fluid flow is diverged over thelength of the splitter vane 30. It will however be understood that thesecond side 38 can instead be the “convergent” side while the first side36 can be the “divergent” side, without departing from the scope of thepresent disclosure.

The first side 36 of the splitter vane 30 faces toward either one of theinner and outer walls 28 a,28 b in order to form a convergent flowpassage 37 in which fluid flow converges, over the length of thesplitter vane 30 in the direction 27 of fluid flow F, toward thecorresponding inner or outer wall 28 a,28 b. In the embodiment shown inFIG. 3, the first side 36 converges towards the inner wall 28 a, therebyforming the convergent flow passage 37 within the internal flow passage29 of tubular body 22 at the curved portion 28. The convergent flowpassage 37 may be defined between the first side 36 and the inner wall28 a of the tubular body 22, over the length of the splitter vane 30between its leading and trailing edges 32,34. This convergent flowpassage 37 becomes narrower over the length of the splitter vane 30, inthe direction 27 of fluid flow F, because the cross-sectional area ofthe convergent flow passage 37 decreases from the inlet of theconvergent flow passage 37 to its outlet (i.e. defined at the leadingedge 32 and the trailing edge 34 of the splitter vane 30).

The second side 38 faces toward either one of the inner and outer walls28 a,28 b, in order to diverge fluid flow within the divergent flowpassage 39 defined between the second side 38 of the splitter vane andthe facing wall of the tubular body 22, and this over the length of thesplitter vane 30. In the embodiment shown in FIG. 3, the second side 38of the splitter vane 30 diverges away from the outer wall 28 b, therebydiffusing fluid flow through this divergent flow passage 39. The secondside 38 therefore defines the divergent flow passage 39 within thetubular body 22 at the curved portion 28. The divergent flow passage 39may be defined between the second side 38 and the outer wall 28 b of thetubular body 22, over the length of the splitter vane 30 between itsleading and trailing edges 32,34. This divergent flow passage 39diverges or becomes larger in cross-sectional area over the length ofthe splitter vane 30 because its cross-sectional area increases from theinlet of the divergent flow passage 39 to its outlet (i.e. defined atthe leading edge 32 and the trailing edge 34 of the splitter vane 30).

The convergent passage 37 formed by the splitter vane 30 thereforeeffectively forms a nozzle, whereby fluid flow within the convergentpassage 37 is accelerated therethrough in the direction of flow. On theopposite side of the splitter vane 30, however, fluid is diffusedthrough the divergent flow passage 39 in the direction of flow.Accordingly, the splitter vane 30 splits the main fluid flow F withinthe internal fluid passage 29 upstream of the splitter vane 30 into twoseparate streams, one on each side of the splitter vane 30. The streamwithin the convergent passage 37 formed on one side of the splitter vane30 is accelerated, whereas the stream within the divergent passage 39 onthe opposite side of the splitter vane 30 is decelerated. As seen in theembodiment of FIG. 3, the size (i.e. in cross-sectional area) of theconvergent passage 37 is however small than that of the divergentpassage 39. It can thus be appreciated that “converging fluid flow” isunderstood herein to refer to accelerating fluid flow through anarrowing passage (namely the convergent passage 37) on one side of thesplitter vane 30. While the overall effect of the diffuser pipe 20 is todiffuse, and thus decelerate the flow F flowing therethrough, theacceleration of the smaller fluid flow stream within the convergent flowpassage 37, defined by the splitter vane 30, is thought to enhance theability of fluid flow ability to remain attached downstream of the bend28 in the diffuser pipe 20. This accelerated fluid flow may also help toencourage, or “bias”, upstream fluid flow towards the convergent flowpassage 37.

Similarly, “diffusing fluid flow” is understood herein to refer toincreasing a static pressure of fluid flow through the broadeningdivergent passage 39 on the opposite side of the splitter vane 30 to theconvergent passage 37. Additional diffusion is understood to occur inthe divergent flow passage 39 of each diffuser pipe 20. Theairfoil-shaped splitter vane 30 therefore defines a pressure side at itsfirst side 36, and a suction side at its second side 38. Structurally,the splitter vane 30 may also act as a stiffener and help to strengthendiffuser pipe 20. Splitter vane 30 can thus be used to replacetraditional stiffening ribs that are sometimes stamped on the wall ofdiffuser pipes.

Referring now to FIGS. 4 and 5, the splitter vane 30 may extend acrossthe diffuser pipe 20 between its inner opposed side walls, orwall-to-wall. This embodiment of the splitter vane 30 divides at leastpart of the curved portion 28 into two separate convergent and divergentflow passages 37,39. In the embodiment shown, the leading edge 32 of thesplitter vane 30 is disposed at a lateral midpoint between opposed innerand outer walls 28 a,28 b, i.e. halfway across the bend of the diffuserpipe 20. In order to define the convergent and divergent flow passages37,39, the trailing edge 34 of the splitter vane 30 is positioned closerto the inner wall 28 a than to the outer wall 28 b.

Referring now to FIGS. 6 and 7, each diffuser pipe 120 may also have anair recirculation conduit 40. The air recirculation conduit 40 isconfigured to supply a compressible fluid (e.g. air) to each of thediffuser pipes 120. It is known that the main gas flow in the diffuserpipes 120 can experience an adverse pressure gradient in the directionof flow F. This pressure gradient coupled with existing friction forcesin the boundary layer of the wall of the diffuser pipes 120 canstrengthen the effect of deceleration experienced by the main gas flow,which may result in the boundary layer being built up within thediffuser pipe 120. This buildup leads to increased flow blockage,diminishes pressure recovery, and can eventually lead to flowseparation.

By recirculating the compressible air into the diffuser pipes 120 at asuitable location, it may be possible to prevent and/or reduce increasedblockage and flow separation by energizing the boundary layer along thewalls of the diffuser pipes 120. Flow with momentum deficit at the wallsis replaced with high momentum flow, making the main fluid flow moreresistant to flow separation. Another possible benefit may be that therecirculated compressible air helps to keep the main fluid flow attachedto the walls. Each air recirculation conduit 40 draws the air from asupply 42 of pressurized air. Each air recirculation conduit 40 extendsbetween a conduit inlet 44 which draws air from the supply 42, and aconduit outlet 46 which communicates with a corresponding diffuser pipe120 to introduce the pressurized air into the diffuser pipe 120.

The supply 42 can be any source of the compressible air. Thecompressible fluid from this supply 42 can be actively provided, meaningthat it can be pumped or otherwise actively directed to each airrecirculation conduit 40. Alternatively, the supply 42 can simply be aregion of higher pressure air within the compressor or downstreamthereof. For example, the supply 42 of compressible air can be theregion downstream of the outlets of the diffuser pipes 120 and adjacentto an inlet of the combustor. This area will generally be filled withcombustion chamber inlet air, or so-called “P3” air. Therefore, thecompressible fluid injected into the diffuser pipes 120 via the airrecirculation conduits 40 can be P3 air. In such a configuration, the P3compressible fluid can recirculate passively toward the airrecirculation conduits 40 because the static pressure at the supply 42is typically greater than the static pressure at the location of the airrecirculation conduits 40. Such a passive circulation system can be moreeasily implemented in existing diffusers. In most embodiments, thecompressible fluid is the same as the fluid of the main fluid flow.

It can thus be appreciated that each air recirculation conduit 40 can bea pipe or duct, or can alternatively be a bore, orifice, or slot in thewall of a corresponding diffuser pipe 120. The conduit outlet 46 opensinto, and is in fluid communication with, a corresponding diffuser pipe120. Each conduit outlet 46 can be an recirculation slot or holeextending through the wall or the diffuser pipe 120. The conduit outlet46 may open into the diffuser pipe 120 at a point upstream of thesplitter vane 30, so that the air recirculation conduit 40 canrecirculate the compressible fluid into the diffuser pipe 120 at alocation upstream of the splitter vane 30. Indeed, the conduit outlet 46and air recirculation conduit 40 may recirculate air directly into theconvergent flow passage 37. The converging/diverging configurationwithin the diffuser pipe 30, which is created by the presence of thesplitter vane 30, as well as the upstream air recirculation conduit 40,help to create an “air ejector” effect within the diffuser pipe 20.

It is contemplated that by locating the conduit outlet 46 upstream ofthe splitter vane 30, the compressible fluid exiting the airrecirculation conduit 40 may energize the boundary layer of the mainfluid flow in the diffuser pipe 120 so as to reduce or prevent any flowseparation. The recirculated air may prevent flow blockage and flowseparation by energizing the boundary layer at the walls 28 a,28 b ofthe diffuser pipe 120. Flow with a momentum deficit at the walls 28 a,28b is replaced with a higher momentum flow, making the fluid flow moreresistant to flow separation. It is believed that such a reduction inflow separation can reduce the mixing losses in the diffuser pipe 120,improve the overall efficiency and range of the compressor, and improvethe operability of the front stages of the gas turbine engine.

The number of conduit outlets 46 that an air recirculation conduit 40has may be greater than one, such that the air recirculation conduit 40can recirculate the compressible fluid into the diffuser pipe 120 atmultiple locations on the diffuser pipe 120.

Referring to FIG. 3, there is also disclosed a method for diffusingfluid flow. The method includes conveying fluid flow through a divergingtubular body 22.

The method also includes splitting fluid flow at the curved portion 28of the tubular body 22 so as to divide fluid flow between a firstconvergent fluid passage 37, and a second fluid passage 39. Fluid flowis converged in the first fluid passage 37, and may be diffused in thesecond fluid passage 39.

Because of the diffusion process, the diffuser pipes 20,120 experienceadverse pressure gradients in the direction of flow F, with end wallboundary layer being built up as the result. The buildup may lead toincreased blockage, diminished pressure recovery and eventually lead toflow separation. The flow separation usually starts at the diffuser bend28 where the curvature is at its maximum. The splitter vane(s) 30 mayreduce pressure gradient across the curved portion 28 and help the flowF to negotiate the tight turn more efficiently. The airfoil splittervanes 30 described herein may also facilitate swirl removal.Computational fluid models can be used to optimize the splitter vane 30length and/or location, while the inner and outer walls 28 a, 28 b, canbe shaped in accordance with the splitter vane 30 to best conform to astator pitch.

The diffuser pipes 20,120 with splitter vane(s) 30 at the curvedportions 28 thereof may limit flow separation and/or reduce thelikelihood of it initiating. Since mixing losses may be a prominentcontributor to diffuser pipe loss and is initiated mostly at the curvedportion 28, employing splitter vane(s) 30 at that location may be moreeffective than anywhere else in the diffuser pipe 20.

Although the embodiments described above include only a single splittervane 30, it is to be understood that the diffuser pipe 20 as describedherein may in fact include two or more such splitter vanes 30,configured as required to form several different converging flowpassages 37, etc. within the confines of the larger diffusing fluid flowpassage 29 of the diffuser pipe 20. This may be desirable, for example,in the case where the diffuser pipe 20 has several different bends 28 init, in which case it a splitter vane 30 may be disposed within theinternal fluid passage proximate each of the bends, in order to form aconvergent passage 37 near the bends which locally accelerates the flowto reduce the likelihood of occurrence of flow separation at ordownstream of the bend.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed without departing from the scope of the invention disclosed.Other modifications which fall within the scope of the present inventionwill be apparent to those skilled in the art, in light of a review ofthis disclosure, and such modifications are intended to fall within theappended claims.

1. A compressor diffuser for a gas turbine engine comprising: at leastone diffuser pipe having a tubular body defining an internal flowpassage extending therethrough, the tubular body including a firstportion extending in a first direction, a second portion extending in asecond direction different from the first direction, and a curvedportion fluidly linking the first portion and the second portion; and asplitter vane disposed within the internal flow passage of the curvedportion of the tubular body, the splitter vane defining a convergentflow passage between itself and a radially inner wall of the curvedportion.
 2. The compressor diffuser of claim 1, wherein the splittervane defines a divergent flow passage between itself and a radiallyouter wall of the curved portion.
 3. The compressor diffuser of claim 1,wherein the at least one diffuser pipe has an air recirculation conduitextending between a conduit inlet configured to receive a flow ofcompressed air from a supply, and a conduit outlet communicating withthe internal flow passage to introduce air therein.
 4. The compressordiffuser of claim 3, wherein the recirculation conduit fluidly links anarea downstream of the splitter vane with an area upstream of a trailingedge of the splitter vane.
 5. The compressor diffuser of claim 3,wherein the conduit outlet of the recirculation conduit opens into theinternal flow passage upstream of the splitter vane.
 6. The compressordiffuser of claim 3, wherein the supply is disposed downstream of the atleast one diffuser pipe in a region of the compressor having combustionchamber inlet air, the combustion chamber inlet air having a staticpressure greater than a static pressure of air at the conduit inlets ofthe recirculation conduit, the combustion chamber inlet air circulatingpassively from the supply to the conduit inlet of the recirculationconduit.
 7. The compressor diffuser of claim 1, wherein the splittervane extends between opposed inner side walls of the curved portionacross the internal flow passage.
 8. A method for diffusing fluid flowin a compressor, comprising: conveying fluid flow through a diverginginternal flow passage of a compressor diffuser, the internal flowpassage including at least one curved portion; splitting the fluid flowinto first and second fluid passages defined within the internal flowpassage, the first and second fluid passages being at least partiallylocated within the curved portion; and converging fluid flow through thefirst fluid passage.
 9. The method of claim 8, further comprisingdiffusing fluid flow through the second fluid passage.
 10. The method ofclaim 8, wherein converging fluid flow includes reducing across-sectional area of the first fluid passage along a length thereof.11. The method of claim 10, wherein diffusing fluid flow includesincreasing a cross-sectional area of the second fluid passage along alength thereof.
 12. The method of claim 8, further comprisingrecirculating additional air into the conveyed fluid flow within theinternal flow passage.
 13. The method of claim 12, wherein recirculatingadditional air comprises recirculating the additional air into the firstfluid passage.
 14. The method of claim 12, wherein recirculatingadditional air includes recirculating air into the internal flow passageupstream of splitting fluid flow.
 15. The method of claim 12, whereinrecirculating additional air into the internal flow passage includesdrawing compressed air from a supply of combustor inlet air downstreamof the first fluid passage.
 16. The method of claim 15, whereinrecirculating additional air includes passively recirculating thecombustor inlet air into the internal flow passage.
 17. A centrifugalcompressor, comprising: an impeller having an inner hub with a pluralityof vanes extending therefrom, the impeller being rotatable within anouter shroud about a central longitudinal axis, the impeller having aradial impeller outlet; and a diffuser configured to diffuse gasreceived from the impeller outlet, the diffuser comprising: at least onediffuser pipe having a tubular body defining a flow passage extendingtherethrough, the tubular body including a first portion extending in afirst direction, a second portion extending in a second directiondifferent from the first direction, and a curved portion fluidly linkingthe first portion and the second portion; and a splitter vane disposedwithin the internal flow passage of the curved portion of the tubularbody, the splitter vane defining a convergent flow passage betweenitself and a radially inner wall of the curved portion.
 18. Thecentrifugal compressor of claim 17, wherein the splitter vane defines adivergent flow passage between itself and a radially outer wall of thecurved portion.
 19. The centrifugal compressor of claim 17, wherein theat least one diffuser pipe has an air recirculation conduit extendingbetween a conduit inlet configured to receive a flow of compressiblefluid from a supply, and a conduit outlet communicating with the atleast one diffuser pipe to introduce air therein.
 20. The centrifugalcompressor of claim 19, wherein the conduit outlet of the injectionconduit opens into the at least one diffuser pipe upstream of thesplitter vane.